Abstract: A single-beam optical tweezers experimental system is presented，which can be used to trap some tiny particles stably and also realize microscopic imaging. Based on this system， the dynamic capture process of single silica microspheres is studied.It is found from experiments that the focal plane of the objective lens and the imaging plane of the CCD must be in a same plane when the particles are captured and imaged at the same time.In addition， the effect of the numerical aperture of the focusing objective lens and the power of the laser for stable trapping of the particles are also found in the experiment.Both experimental results and theoretic analyses indicated that the larger of the numerical aperture and the greater of the laser power leaded to enlarge the capture force of the particles，so the particles can be stably captured.This study is helpful in the field of college physics teaching and biological micro-manipulation.

Key words: optical tweezers, high numerical aperture objective lens, stable capture, silica microspheres